The present invention relates to a multiband detector used to generate a plurality of sub-band symbols from a distored multiband signal applied to a detector input thereof.
Such a multiband detector is already known in the art, e.g. from the article xe2x80x98A Multicorrier E1-HDSL Transceiver System with Coded Modulationxe2x80x99 from the authors Peter S. Chow, Noofal Al-Dhchir, John M. Cioffi and John A. C. Bingham. This article was published in the issue Nxc2x0 3, May/June 1993 of the Journal of European Transactions on Telecommunications and Related Technologies (ETT), pages 257-266.
Therein, the multiband detector, named DMT (Discrete Multi Tone) receiver and drawn in FIG. 5 of that article, contains an arrangement which reconstructs the individual carrier signals of an incoming multicarrier signal and which compensates for distortion of the incoming multicarrier signal due to transmission over a transmission line in between a DMT transmitter drawn in FIG. 4 of that article and the DMT receiver drawn in FIG. 5 of that article. This arrangement consists of the cascade connection of a time domain equalizer, a serial to parallel converter with cyclic prefix stripper, a fast fourier transformer and a frequency domain equalizer. The time domain equalizer is a short adaptive finite impulse response filter which aims to reduce the cyclic prefix of multicarrier symbols by shortening the impulse response length of the transmission line. The time domain equalizer so helps to reduce intersymbol interference with an acceptable cyclic prefix length. Samples of a single multicarrier symbol in the incoming multicarrier signal are then paralleled by the serial to parallel converter and applied to the fast fourier transformer to be transformed from time domain to frequency domain. Since the equalized channel, i.e. the combination of transmission line and time domain equalizer, is not yet flattened, a frequency domain equalizer is included in the arrangement to compensate for phase and amplitude distortions of individual carriers. The frequency domain equalizer thereto consists of a parallel structure of one-tap filters, adaptive via a least mean square technique. The arrangement of time domain equalizer, serial to parallel converter, fast fourier transformer and frequency domain equalizer is followed by what is called a decoder in the cited article but is named a decision unit in this document. This decoder or decision unit compares the single carrier signals at the outputs of the frequency domain equalizer with the constellation schemes used to modulate the respective carriers and derives therefrom the symbols modulated on the different carriers.
An arrangement which compensates for distortion in a discrete wavelet multitone signal, i.e. another kind of multiband signal, and which reconstructs the different wavelet bands from the distorted wavelet multitone signal is known from the article xe2x80x98Overlapped Discrete Multitone Modulation for High Speed Copper Wire Communicationsxe2x80x99 from the authors Stuart D. Sandberg and Michael A. Tzonnes. This article was published in the IEEE Journal on selected Areas in Communications, Vol. 13, Nxc2x0 9 of December 1995, and the arrangement described therein and drawn partially in FIG. 1 has an architecture similar to the above described one. A pre-detection equalizer suppresses intersymbol interference by digitally filtering the incoming discrete wavelet multitone signal, a wavelet transformer generates the wavelet sub-band signals and applies these wavelet sub-band signals in parallel to a post-detection equalizer which again consists of a parallel structure of single band equalizers adapted via a least mean square technique. The arrangement, except for the length in taps of the pre-detection equalizer and post-detection equalizer and the nature of the transformation used to reconstruct the sub-bands from the multiband signal, does not differ significantly from the above described arrangement with time domain equalizer, fast fourier transformer and frequency domain equalizer. The arrangement in the article from Stuart D. Sandberg and Michael A. Tzonnes further is coupled to a so called constellation symbol decision unit comparable to the decoder in the article from Peter S. Chow et al.
In a more general article xe2x80x98Multicarrier Modulation for Data Transmission: An Idea Whose Time Has Comexe2x80x99, the author John A. C. Bingham proposes a structure with a simple equalizer which performs a time domain convolution, a transformer which reconstructs orthogonal sub-bands from a multiband signal, and a set of parallel single band equalizers. The article from John A. C. Bingham was published in IEEE Communications magazine of May 1990 and obviously suggests to use, whatever the nature of the multiband to sub-band transformation, a detector with an architecture similarly to that of the above cited articles.
The architecture of a detector for sub-band reconstruction and distortion compensation, known from the above reference articles, has the disadvantage that it is insufficiently flexible vis a vis changes in the distortion of the multiband signal on the transmission line. Narrowband distortions for instance, which affect only a few sub-bands and which are likely to occur as can be derived from the articles of John A. C. Bingham (see page 12, paragraph entitled xe2x80x9cSingle-Frequency Interferencexe2x80x9d) and Peter S. Chow et al (see page 259; right-hand column, lines 28-29), can be compensated for by adapting the taps of the pre-detection equalizer or by enlarging the number of taps in the pre-detection equalizer but these solutions obviously have an influence on the detection of other sub-bands which are not affected by the norrowband distortions. This solution consequently is not very effective which explains why Peter S. Chow and John A. C. Bingham propose to either update the bit allocations or to avoid using affected carriers in their respective articles in response to narrowband distortions.
The known architecture moreover limits the applicability of the multiband detector to environments wherein one and the some kind of multiband signal is transferred. By the choice of the sub-band reconstructor in between the pre-detection equalizer and the post-detection equalizer, the multiband detector becomes able to either receive DMT (Discrete Multi Tone) signals whose sub-bands are reconstructed via a fast fourier transformation, DWMT (Discrete Wavelet Multi Tone) signals whose sub-bands are reconstructed via a wavelet transformation, or another kind of multiband signal whose orthogonal sub-bands are reconstructed via yet another transformation. Receiving another kind of multiband signal with a detector having the known architecture requires replacement of components therein. For evident reasons (complexity of interfacing) this is not done.
An object of the present invention is to provide a multiband detector of the above known type, but which is more flexibly adaptive to distortion changes on the transmission line, even if these changes in distortion affect only a few sub-bands, and which is suitable for detection of different kinds of multiband signals.
This object is realised by a multiband detector used to generate a plurality of sub-band symbols from a distorted multiband signal applied to a detector input thereof, the multiband detector comprising the cascade connection of: a sub-band reconstruction and distortion compensation arrangement whose arrangement input is coupled to the detector input and which is adapted to compensate for distortion in the distorted multiband signal and to reconstruct a plurality of sub-band signals from the distorted multiband signal, and to source each sub-band signal amongst the plurality of sub-band signals via a respective arrangement output amongst a plurality of arrangement outputs; and a decision unit with a plurality of unit inputs coupled one by one to the plurality of arrangement outputs and a plurality of associated unit outputs, the decision unit including between each unit input and associated unit output a comparator means adapted to compare the sub-band signal with a constellation scheme and to thereupon decide upon the value of a sub-band symbol amongst the plurality of sub-band symbols, wherein the sub-band reconstruction and distortion compensation arrangement contains between the arrangement input and each arrangement output, a single digital filter whose taps are set to perform sub-band reconstruction and distortion compensation simultaneously and to thereby generate a said sub-band signal.
Indeed, according to the present invention, the multiband detector is given a more homogeneous architecture over the different sub-bands. The different input-output paths of the detector, which each reconstruct one sub-band signal out of the incoming multiband signal, are very similar and can be tuned separately. By adapting the taps of one digital filter or increasing/decreasing the number of taps of one digital filter, the corresponding sub-band is made less or more distortion resistant independently from the other sub-bands.
Furthermore, by adapting the taps of all digital filters, the detector according to the present invention can be enabled to receive multiband signals of different nature. This is so because the taps of each digital filter contain two contributions. A first contribution depends on the set of orthogonal base functions used to constitute the multiband signal and allows reconstruction of the sub-bands. The second contribution depends on the transmission line characteristics and allows to compensate for distortion due to transmission of the multiband signal over the transmission line. If the first contribution is modified to accord with a new set of base functions, the detector is able to receive for instance DWMT signals instead of DMT signals. Such a modification of the taps is realisable according to the present invention. The detector according to the present invention hence may be used to manufacture a multi-mode multiband receiver which, when switched from one mode to another, is able to receive another kind of multiband signal.
It has to be noticed that the term xe2x80x98comprisingxe2x80x99, used in the claims, should not be interpreted as being limitative to the means listed thereafter. Thus, the scope of the expression xe2x80x98a device comprising means A and Bxe2x80x99 should not be limited to devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B.
Similarly, it has to be noted that the term xe2x80x98coupledxe2x80x99, also used in the claims, should not be interpreted as being limitative to direct connections only. Thus, the scope of the expression xe2x80x98a device A coupled to a device Bxe2x80x99 should not be limited to devices or systems wherein an output of device A is directly connected to an input of device B. It means that there exists a path between an output of A and an input of B which may be a path including other devices or means.
An additional feature of the present multiband detector is such a detector that further comprises a control unit adapted to modify the taps of the digital filter upon changes in distortion of the distorted multiband signal.
Indeed, although changes in the distortion on the transmission line may be compensated for by updating the allocation of bits to different sub-bands, or may be updated by a technician which resets the tap values of the digital filters based on signal-to-noise ratio measurements on the transmission line, it is advantageous to incorporate a control unit which automatically adapts the filter taps when changes in the distortion on the transmission line are detected. The transmission line thereto may be monitored continuously or at discrete time intervals.
Another feature of the multiband detector according to the present invention is such a detector with a control unit that is adapted to modify the amount of the taps of the digital filter.
In this way, the control unit is not only capable of adapting the tap values but also of modifying the number of taps in a digital filter rendering a higher degree of flexibility to react to channel changes.
A further advantageous feature of the multiband detector according to the present invention, is such a detector where the comparator means is coupled to at least one tapped delay line adapted to feed back a linear combination of subsequent sub-band symbols at the corresponding unit output to another unit input than the unit input to be added there to a sub-band signal received from the sub-band reconstruction and distortion compensation arrangement.
In this way, intercarrier interference and intersymbol interference can be reduced in a less processing intensive manner than it is compensated for by the sub-band reconstruction and distortion compensation arrangement. This is so because the decision unit operates at the multiband symbol clock speed whereas the sub-band reconstruction and distortion compensation arrangement operates at the sample clock speed. Modification of a tap of a digital filter allows access to a greater frequency bandwidth than modification of a coefficient in a tapped delay line since the tops in the digital filters have a higher time resolution than the coefficients in the tapped delay lines. Hence, there is a trade-off between accuracy and mathematical complexity which determines the length of the tapped delay lines in the decision unit and of the digital filters in the sub-band reconstruction and distortion compensation arrangement.
Yet another advantageous feature of the multiband detector according to the present invention is where the control unit further is adapted to modify coefficients in the linear combination.
A similar reasoning to the one set out above for claim 2 in relation with adaptation of the taps of the digital filters leads to the conclusion that it is advantageous to have the control unit automatically updating the coefficients in the tapped delay lines of the decision unit when changes in the distortion on the transmission line are detected.
Furthermore, a feature of the present invention is where the detector has a sample clock of the multiband detector which has a higher sample clock speed than a sample clock of a multiband generator which generated the multiband signal.
In this way, the available bandwidth for sub-band reconstruction is increased significantly which has an improving effect on the sub-band reconstruction process, especially if the base functions of the multiband signal have a lot of their energy dispersed in sidelobes. The more energy is located at high frequencies and the flatter the channel attenuation, the more useful it is to sample the received multiband signal pie rate.